1. Field of the Invention
The present invention relates to an image processing apparatus for obtaining image signal by tonal processing of an input image signal.
2. Related Background Art
For the purpose of obtaining tonal rendition in image formation for example with a laser beam printer by binary encoding of a digital image signal, the present applicant already proposed, in the U.S. patent application Ser. No. 765,938, a process of converting said digital image signal into an analog signal and comparing thus converted signal with a pattern signal such as a triangular signal, thereby obtaining a pulse-width modulated binary signal.
In such process, the level of the analog video signal and that of the triangular wave may assume a relationship as represented by (a), (b) or (c) in FIG. 18, for example according to the nature of image desired.
For example in a situation represented by (a) in FIG. 18, the tonal rendition in the low image density area is significantly affected if there exists even a small change in the relationship between the white level .alpha. of the analog video signal and the maximum value of the triangular wave, for example due to the thermal characteristic of the triangular wave generator. Also the tonal rendition in the high image density area is significantly affected if there exists even a small change in the relationship between the black level .beta. of the analog video signal and the minimum value of the triangular wave. Also even if there is no change in the relationship between the white and black levels of the analog video signal and the triangular wave, the regulation of said relationship is extremely difficult even with a synchroscope because even a minute aberration significantly affects the resulting image. Furthermore, even if the relationship between the white and black levels of the analog video signal and the triangular signal is maintained constant, the tonal rendition is significantly affected particularly in the high and low image density areas of the image because, in case of a laser beam printer, of fluctuations or variations in the laser power, response characteristic of laser, E-V characteristic of the photosensitive member etc.